1. Field of the Invention
The present invention relates to a radiation image storage panel employable in a radiation image recording and reproducing method utilizing a stimulable phosphor, and a process for the preparation of the same. Further, this invention relates to a radiographic intensifying screen and a process for the preparation of the same.
2. Description of Prior Arts
In a variety of radiography such as medical radiography for diagnosis and industrial radiography for nondestructive inspection, a radiographic intensifying screen is employed in close contact with one or both surfaces of a radiographic film such as X-ray film for enhancing the radiographic speed of the system. The radiographic intensifying screen consists essentially of a support and a phosphor layer provided thereon. Further, a transparent film is generally provided on the free surface of the phosphor layer to keep the layer from chemical deterioration and physical shock.
The phosphor layer comprises a binder and phosphor particles dispersed therein. The phosphor layer is generally provided on a support under an atmospheric pressure utilizing the following coating procedure.
The phosphor particles and the binder are mixed in an appropriate solvent to prepare a coating dispersion. The coating dispersion is directly applied onto a surface of a support for radiographic intensifying screen under an atmospheric pressure using a doctor blade, roll coater, knife coater or the like, and the solvent contained in the coating dispersion applied is removed to form a phosphor layer. Alternatively, the phosphor layer is provided on the support by applying the coating dispersion to form a phosphor sheet, peeling off the sheet from the false support, and then causing the sheet to adhere to the genuine support.
When excited with a radiation such as X-rays passing through an object, the phosphor particles contained in the phosphor layer emit light of high luminance in proportion to the dose of the radiation. Accordingly, an X-ray film placed in close contact sufficiently to form a radiation image of the object, even if the radiation is applied to the object at a relatively small dose.
It is desired for the radiographic intensifying screen having the above-mentioned basic structure to exhibit a high radiographic speed and to provide an image of high quality (high sharpness and high graininess).
The radiographic speed of the radiographic intensifying screen is essentially determined by the total amount of emission given by the phosphor contained therein, and the total amount varies depending upon not only the emission luminance of the phosphor but also the content (i.e., amount) of the phosphor in the phosphor layer. The large content of the phosphor also results in increase of absorption of a radiation such as X-rays, so that the screen shows high radiographic speed and provides an image of improved quality, especially graininess. On the other hand, assuming that the content of the phosphor layer is kept at the same level, a screen utilizing such a phosphor layer provides an image of high sharpness if the phosphor layer is densely packed with the phosphor, because such phosphor layer can be made thinner to reduce spread of stimulating rays caused by scattering in the phosphor layer.
The present applicant has already applied for patent with respect to a radiographic intensifying screen having a phosphor layer in which the phosphor is densely packed. The phosphor layer of the application is compressed to lower the void ratio (U.S. Pat. No. 4,952,813; European Patent Application No. 83108541.0). In the specification of the application, disclosed are examples of resins employable for the binder of the phosphor layer, for example, natural polymers such as proteins (e.g., gelatin), polysaccharides (e.g., dextran) and gum arabic; and synthetic polymers such as polyvinyl butyral, polyvinyl acetate, nitrocellulose, ethyl cellulose, vinylidene chloride-vinyl chloride copolymer, polyalkyl (meth)acrylate, vinyl chloride-vinyl acetate copolymer, polyurethane, cellulose acetate butyrate, polyvinyl alcohol, and linear polyester.
The phosphor density in the phosphor layer of the above-mentioned screen is heightened by compressing the phosphor layer, and an image provided by the screen exhibits improved sharpness as compared with one obtained by a conventional radiographic intensifying screen. However, with respect to the radiographic speed and the graininess, some of the above-mentioned screens rather deteriorate.
As a method replacing a conventional radiography using a radiographic intensifying screen, a radiation image recording and reproducing method utilizing a stimulable phosphor is described, for instance, in U.S. Pat. No. 4,239,968 and is practically used. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method involves the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at a considerably smaller dose, as compared with the conventional radiography using a combination of a radiographic film and a radiographic intensifying screen.
The radiation image storage panel employed in the above-described method generally comprises a support and a stimulable phosphor layer provided on one surface of the support. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a transparent film is generally provided on the free surface of the phosphor layer to keep the layer from chemical deterioration and physical shock.
The stimulable phosphor layer generally comprises a binder and stimulable phosphor particles dispersed therein, and the stimulable phosphor emits light (stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the stimulable phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
As described hereinbefore, the radiation image recording and reproducing method is very advantageous for obtaining a radiation image as a visible image. It is desired for the radiation image storage panel employed for the method, as well as a radiographic intensifying screen employed for a conventional radiography, to have a high sensitivity and provide an image of high quality (high sharpness and high graininess).
The sensitivity of the radiation image storage panel is essentially determined by the total amount of stimulated emission given by the stimulable phosphor contained therein, and the total amount varies depending upon not only the emission luminance of the phosphor but also the content (i.e., amount) of the phosphor in the phosphor layer. The large content of the phosphor also results in increase of absorption of a radiation such as X-rays, so that the panel shows an increased high sensitivity and provides an image of improved quality, especially graininess. On the other hand, assuming that the content of the phosphor layer is kept at the same level, a panel utilizing such a phosphor layer provides an image of high sharpness if the phosphor layer is densely packed with the phosphor, because such phosphor layer can be made thinner to reduce spread of stimulating rays caused by scattering in the phosphor layer.
The present applicant has already applied for patent with respect to a radiation image storage panel having a phosphor layer in which the stimulable phosphor is densely packed. The phosphor layer of the application is compressed to lower the void ratio (U.S. Pat. No. 4,910,419; European Patent Application No. 011365.6). In the specification of the application, disclosed are examples of resins employable for the binder of the phosphor layer, for example, natural polymers such as proteins (e.g., gelatin), polysaccharides (e.g., dextran) and gum arabic; and synthetic polymers such as polyvinyl butyral, polyvinyl acetate, nitrocellulose, ethyl cellulose, vinylidene chloride-vinyl chloride copolymer, polyalkyl (meth)acrylate, vinyl chloride-vinyl acetate copolymer, polyurethane, cellulose acetate butyrate, polyvinyl alcohol, and linear polyester.
The phosphor density in the phosphor layer of the above-mentioned panel is heightened by compressing the phosphor layer, and an image provided by the panel exhibits improved sharpness as compared with one obtained by a known radiation image storage panel. However, with respect to the graininess, some of the above-mentioned panels rather deteriorate.